Our proposal will address the need for a safe and effective HIV vaccine using next generation adenovirus (Ad) - based vectors. A variety of viral vectors have been employed to accomplish gene-based vaccination. One particularly promising approach is based upon genetic transduction via replication-incompetent adenoviral vectors (Ad). This utility has predicated the application of Ad-based vectors for a wide range of vaccine approaches, including recent studies demonstrating utility for Malaria, Ebola, SARS, Pseudomonas, and HIV. However, the recent findings of the STEP HIV vaccine clinical trial failed to reproduce promising findings seen in animal model systems. In this regard, this trial, which used a Ad5-based vaccine, failed to protect Ad5-seronegative individuals against infection and may even have enhanced infection in vaccinees with prior immunity to adenoviruses. In the aggregate, the general agreement is that efforts to reduce vector immunogenicity with respect to pre-existing Ad5 immunity will be required to generate a safer and effective Ad- based vector as an HIV vaccine. Therefore, our proposal will address these key points by engineering multiple genetic alterations as follows: (1) To evade humoral responses to the Ad5 vector, the major capsid protein hexon will be exchanged for the hexon of Ad3, a less prevalent serotype. Human immune responses largely focus on epitopes present in the hypervariable regions (HVR) of the Ad capsid hexon; therefore, a chimeric Ad5H3 capsid will evade recognition by preimmune Abs to Ad5 hexon (2) To induce HIV specific immune responses we will employ HIV capsid incorporation of antigen. We hypothesize that these Ad vectors can circumvent Ad5 immunity and provide HIV-specific immunity. The full merit of the promising adenovirus-based approach for vaccination is currently limited by host related immunity toward the vector. Accomplishing the goals of this proposal will expand our current understanding of host-vector interactions that will be critical for advancing the Ad vector platform as a viable approach for developing effective HIV vaccines. .